The invention relates to a system for measuring blood pressure during a limited number of blood pressure cycles at an area overlying an artery of a living subject.
A system of the type defined in the opening paragraph is known from U.S. Pat. No. 4,660,566.
The known system comprises a measuring unit comprising a sensor provided on an inflatable cushion, and a processing unit. The cushion is provided on a carrier comprising a bracelet of flexible material worn by a user around his or her wrist. In this known system, pressure pulses are applied by means of the cushion to a local, discrete external point overlying the radial artery of the subject, to effect a blocking and successively an unblocking condition of the artery, or to effect an unblocking and successively a blocking condition of the artery. When the sensor detects said respective unblocking or blocking condition, the instantaneous value of the pressure in the cushion is measured for the purpose of providing through the processing unit a value of said subject""s blood pressure. A disadvantage of the known system is that the measurement results of the systolic and diastolic blood pressure given by the system are not accurate due to the fact that the measurements along the subjects pressure line cannot be exactly located, and it is not known whether there is at least one measurement at the systolic peak and one at the diastolic minimum of the blood pressure cycle. Because of this, long measurement times are needed in order to have a high statistical probability that the measurements have taken place at those points in time at which the systolic and the diastolic pressure occurred.
It is an object of the invention to provide an improved system for measuring blood pressure which offers a more accurate and faster measurement of systolic and diastolic blood pressures.
To achieve this object, a system according to the invention is characterized in that it comprises:
means for applying a pressure to the artery for occluding said artery in response to a pressure signal and for subsequently releasing said pressure to re-open said artery in response to a pressure release signal;
means for determining a reference point in time, and for providing a reference time signal corresponding to said reference point in time,
means for detecting a first point in time at which a maximum pressure occurs during a blood pressure cycle, and for providing a first time signal corresponding to said first point in time;
means for computing, from the reference time signal and the first time signal, a first time duration from the reference point in time to the first point in time;
means for detecting a second point in time at which an opening of the artery occurs after the release of said pressure, and for providing a second time signal corresponding to said second point in time;
means for determining the pressure in said means for applying pressure to the artery, at the second point in time;
means for computing, from the reference time signal and the second time signal, a second time duration from the reference point to the second point in time;
means for computing, by comparing the first time duration with the second time duration, a delay time from said reference point in time after which a next pressure release signal is to be supplied to said means for applying a pressure, and for computing a pressure to be applied to said artery in a next measuring cycle, and for supplying subsequently a pressure signal and a pressure release signal corresponding to said computed pressure and delay time to said means for applying a pressure.
The measurement of blood pressure is started by applying a pressure to the artery, until the artery is occluded by said pressure. After that, the reference point in time is determined, from which point in time the pressure in the cushion is subsequently released until the artery re-opens. During this release of pressure, the point in time at which the maximum pressure occurs is detected, and the point in time at which the opening of the artery occurs is detected, and a first and a second time signal corresponding to these respective points in time are provided. The pressure in the pressure applying means which indicates the blood pressure in the artery is determined at the second point in time, being the time of the opening of the artery. The first time duration from the reference point in time to the point in time at which the maximum pressure occurs is then compared with the second time duration from the reference point in time to the point in time at which the opening of the artery occurs. From this comparison the delay time is computed. This delay time is the delay time from the reference point in time, after which a next pressure release signal is supplied to the means for applying the pressure. Furthermore, the pressure to be applied to said artery in a next measuring cycle is computed in dependence on this comparison. Signals, which correspond, to this computed delay time and pressure are then supplied to the means for applying a pressure to the artery. The delay time and the pressure for the pressure applying means are refined by repeating this measurement a limited number of cycles. The comparison of the first time duration with the second time duration is thus repeated until the point in time at which the opening of the artery coincides with the point in time at which the maximum pressure occurs within a pretermined tolerance time interval. The pressure determined in the pressure applying means at this moment gives an accurate indication of the systolic blood pressure of the subject. In this way the systolic blood pressure is derived with accuracy and in a fast manner.
An embodiment of a system in accordance with the invention is characterized in that the reference point in time is a point in time at which a start of a systolic phase in a blood pressure cycle occurs. The measurement starts at a point in time which corresponds to the start of the rise in blood pressure in the artery, and thus the measured pressures are known to be in the vicinity of the maximum pressure, which is the systolic blood pressure. This is advantageous for increasing the speed with which the systolic blood pressure is measured, because only a limited number of measurements is needed for determining the systolic blood pressure.
An embodiment of a system in accordance with the invention is characterized in that the means for detecting the start of said systolic phase comprise a sensor for detecting during operation a point in time at which a pressure pulse in the means for applying a pressure to the artery rises above a predetermined threshold value. The start of the systolic phase is thus determined in an easy manner.
An embodiment of a system in accordance with the invention is characterized in that it comprises said means for computing, which are furthermore designed for determining a further delay time from said reference point in time after which a further pressure release signal is to be supplied to said means for applying a pressure in a next measuring step for measuring the diastolic blood pressure and for supplying a further pressure release signal corresponding to said further delay time, means for determining, from the timing of at least two heart beats of the subject, a heart rate duration from the reference point in time to an end point in time corresponding to an end of one pressure cycle, means for detecting a further point in time at which an opening of the artery occurs after the release of said pressure in response to said further pressure release signal, and for providing a third time signal corresponding to said further point in time, means for computing, from the reference time signal and the further time signal, a third time duration from the reference point in time to the further point in time, said means for computing further determining, by comparing the heart rate duration and the third time duration, a next value for said further delay time after which a next pressure release signal is to be supplied to said means for applying a pressure in said next measuring step, and for supplying subsequently a next value for said further pressure release signal corresponding to said next value for said further delay time to said means for applying a pressure.
For determining the diastolic pressure, a further delay time from said reference point in time, after which a further pressure release signal is to be supplied to said means for applying a pressure, is determined in a next measuring step. A further pressure release signal corresponding to said further delay time is then supplied to the means for applying a pressure, and the pressure in the cushion is subsequently released until the artery re-opens. During this release of pressure, the point in time at which the opening of the artery occurs is detected, and a third time signal corresponding to this point in time is provided. Next to this, a heart rate duration from the reference point in time to an end point in time corresponding to an end of one pressure cycle is determined from the timing of at least two heart beats. This heart rate duration and this third time duration are compared. This comparison provides a next value for the further delay time. A signal which corresponds to this computed next value for the further delay time is then supplied to the means for applying a pressure to the artery. The following next values for the further delay time for the pressure applying means are refined by repeating this measurement a limited number of cycles. The comparison of the heart rate duration with the third time duration is thus repeated until the point in time at which the opening of the artery coincides within a predetermined tolerance time interval with the point in time at which the blood pressure cycle ends, this being the point in time at which the minimum pressure occurs. The pressure determined in the pressure applying means at this moment gives an accurate indication of the diastolic blood pressure of the subject. In this way the diastolic blood pressure is derived with accuracy and in a fast manner.
An embodiment of a system in accordance with the invention is characterized in that said means for detecting said first point in time comprise a pressure sensor for measuring a pressure in the means for applying a pressure to the artery during operation, and a processor for determining said first point in time from a signal provided by the pressure sensor. The pressure sensor measures the pressure in the means for applying a pressure to the artery and provides a signal to the processor in response to this. The processor then determines said first point in time from this signal. In this way the first point in time is determined in a simple manner.
An embodiment of a system in accordance with the invention is characterized in that the means for detecting said second point in time comprise an electro-optical sensor for detecting the presence of blood in the artery and a further processor for determining said second point in time from a signal provided by the electro-optical sensor. The electro-optical sensor is placed near the artery and detects the presence of blood in the artery by checking the absorption of light emitted by the sensor. When the artery opens, blood enters the artery which absorbs said light. The electro-optical sensor detects this change in the absorption of light, and provides a signal to the processor in response to this. The processor then determines said second point in time from this signal.
An embodiment of a system in accordance with the invention is characterized in that said means for computing also determine a pressure-time profile according to which said pressure is released, said means for computing supplying a pressure release signal corresponding to said profile. The pressure-time profile of releasing the pressure in the means for applying pressure can be adjusted in this manner within a limited number of measuring cycles so as to achieve an accurate measurement. Thus a smaller number of measuring cycles is needed, and a faster measurement is achieved.